1. Field of the Invention
The present invention relates to an ultrasonic testing method. The invention particularly relates to an ultrasonic testing method that is suitable for measuring a dimension such as a thickness of a cast having acoustic anisotropy, such as a directionally-solidified material, a single-crystalline material or the like.
2. Description of the Related Art
Traditionally, an ultrasonic testing method using a reflected longitudinal wave or a reflected transverse wave has been used for measurement of a dimension such as the thickness of metal or the like. Such a method is as follows. A pulsed longitudinal wave or a pulsed transverse wave propagates from a front surface of a test object to the inside of the test object. A wave that is reflected from a bottom surface of the test object is received. A time period for propagation of the reflected wave, or the difference between time periods for propagation of multiply-reflected waves, is measured. A distance by which the ultrasonic wave propagates from the bottom surface to the front surface is calculated by multiplying the time period in which the ultrasonic wave propagates from the bottom surface to the front surface by the acoustic velocity. In this manner, the thickness of the test object is calculated.
A velocity of an ultrasonic wave that propagates in a metal material (such as rolled steel or a casting material) that is processed and thereby has a homogeneous crystalline structure is almost constant regardless of a direction in which the ultrasonic wave propagates. The material that has this characteristic is called an isotropic material. In the aforementioned method, the thickness of the isotropic material can be measured with high accuracy.
A cast is constituted by a coarse solidification structure (crystal grains) formed in a process of cooling molten metal. It is generally known that the solidification structure has a characteristic (acoustical anisotropy) in which acoustic properties such as a velocity and attenuation vary with a direction in which an ultrasonic wave propagates.
For example, in an austenitic material such as stainless steel or a Ni-based alloy, crystal grains form a cubic single crystal. In a cooling process, the material is solidified while a crystal orientation of the material is <100> (<100> is a notation that is treated equivalently to [100], [010], [001] and the like). The crystal orientation <100> is called a crystal growth direction. The solidification structure is constituted by a plurality of crystal grains. Thus, the acoustic characteristics of the cast depend on the statistic of characteristics of the plurality of crystal grains. The crystal growth direction is a direction that is parallel to or nearly parallel to the crystal orientation <100>, while the crystal growth direction varies in a range of approximately 15 degrees. In addition, it is known that crystal orientations (of crystal grains) that are perpendicular to the crystal growth direction are random.
When a thickness of a test object (such as a cast) constituted by a coarse solidification structure is to be measured, especially, when a thickness of the test object is to be measured in a direction perpendicular to the crystal growth direction, crystal orientations are random with respect to the direction in which the thickness is measured. Thus, a velocity of an ultrasonic wave varies due to acoustic anisotropy, depending on the direction in which the ultrasonic wave propagates. Therefore, it is difficult to measure the thickness of the test object with high accuracy.
Another method is known in which an effective acoustic velocity is measured in advance using a calibration test body having the same crystal orientations as those of the test object and the thickness of the test object is measured. However, since crystal orientations that are perpendicular to the crystal growth direction <100> are random, it is difficult to reproduce the acoustic velocity in the test object by use of the calibration test body.
In order to perform the measurement with high accuracy, it is necessary to measure crystal orientations of crystal grains forming a solidification structure in advance. Thus, there is a problem that a simple measurement cannot be performed.
In addition, as an ultrasonic testing method for measuring an anisotropic material for which the testing method for an isotropic material is not directly effective, a method in which a transverse ultrasonic wave that has a polarization (or vibration direction) in the same direction as the crystal growth direction of the test object is incident on the test object (refer to, for example, JP-2000-338092-A).